A multi-mode electrically variable transmission is an advantageous new transmission design that has the ability to reduce engine and electric motor losses at low as well as high vehicle speeds. However, depending on the implementation of the mode-changing mechanism, a multi-mode electrically variable transmission has potential disadvantages. For example a multi-mode electrically variable transmission (“multi-mode EVT”) may experience higher transmission spin losses due to clutch drag and multiple planetary gear set friction. Further, the gearing range within some multi-mode EVTs may limit the electric vehicle (“EV”) drive capabilities.
A simplified illustration of “mechanical point chasing” in FIG. 1 shows the rotations per minute (“RPM”) of a first electric motor A, second electric motor B, and engine of a typical prior art single-mode electrically variable transmission plotted against the rotations per minute of the final drive. As can be seen in FIG. 1, the engine RPM (“ENGINE”) remain constant as the final drive RPM increases until the final drive RPM reaches N1. Meanwhile, the RPM of electric motor B (“MOT B”) increases proportionally with the final drive RPM. In contrast to electric motor B, the RPM of electric motor A (“MOT A”) decreases proportionally with the final drive RPM and is equal to zero at a final drive RPM of N1. Once the RPM of electric motor A reaches small negative values (0 rpm used in this simplified example), any further decrease in RPM of electric motor A results in correspondingly increased system losses. Thus, typical single-mode EVTs will not allow electric motor A to operate at (very) negative RPM when the engine is running. The rotation speed of the engine must account for the lack of a further decline (into negative) of RPM of motor A above final drive RPM of N1. Thus, at final drive RPM above N1, the engine RPM must increase proportionally with the increase in the final drive speed. An increase in engine RPM likely results in the engine operating out of its optimum fuel efficiency or power range, or both. Thus, it becomes necessary to design the gearing of the EVT to compromise between adequate highway (i.e., high-speed) and city (i.e., low-speed) performance and efficiency. Thus, a desirable EVT better optimizes engine operation and electric motor losses, while still providing satisfactory city and highway performance and EV functionality, while also being compactly implementable with a low loss mode-change mechanism.
Some EVTs include an input brake which locks the input shaft of the EVT to prevent the input shaft from rotating. The input brake enables both electric motors within the EVT to provide propulsive force when operating in an EV mode. In addition, the input brake improves the drive and regenerative braking efficiency of the EVT by reducing the total system loss by minimizing the sum of the electric motors and engine losses for a given drive system configuration. However, typical prior art EVTs rely upon a separate clutch and/or brake mechanism apart from the EVT to provide input brake functionality. The addition of this separate clutch and/or brake mechanism adds both cost and drag to the EVT.
In a typical prior art EVT, electric motors are utilized to start the fossil fuel powered engine of the vehicle. Thus, the gear ratio between the electric motors, the size of the electric motors of the EVT, and the size of the batteries that power the EVT must be sufficient to provide adequate starting force for the engine even in the worst starting conditions. Thus, compromises often must be made within the EVT to ensure that the motors have adequate torque to start the engine. As a result, the gear ratio between the electric motors, the size of the electric motors, and the size of the batteries that power the EVT may not be ideal for certain vehicle operation. Larger electric motors and batteries may be necessary to start the vehicle engine, thereby adding weight and cost to the EVT.
An EVT generally has limited reverse gear capabilities and relies solely upon one of its electric motors to provide reverse propulsion. This is problematic during situations in which electric battery power or electric motor torque may be limited such as in extremely hot or cold climates. If electric power fails or provides inadequate propulsive force, the vehicle is simply unable to move in reverse. Thus, a desirable EVT harnesses the propulsive force of the engine for reverse gear operation or uses both electric motors of the EVT to provide reverse propulsion if battery power is adequate.
It is, therefore, desirable to provide an EVT that keeps the engine operating within its efficiency and/or power range while also providing satisfactory city and highway performance. It is, therefore, also desirable to provide an EVT with an integrated input brake. It is also desirable to provide an EVT with sufficient torque to start the engine of the vehicle without making other vehicle performance and cost compromises. In addition, it is desirable to provide an EVT in which the engine may provide propulsive force for reverse gear operation or both electric motors of the EVT can be used for reverse gear operation.